Method for controlling a vapour compression system connected to a smart grid

ABSTRACT

A method for controlling operation of a vapour compression system ( 1 ) is provided, the vapour compression system ( 1 ) comprising two or more refrigeration entities, such as display cases. A signal representing a reference power consumption is received and compared to an actual power consumption of the vapour compression system ( 1 ). Based on the comparison, local controllers ( 3 ) calculate a setpoint temperature for a corresponding refrigeration entity, in order to obtain a power consumption which is equal to the reference power consumption. Each refrigeration entity is controlled in accordance with the calculated setpoint temperatures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates by reference subject matter disclosed in the International Patent Application No. PCT/IB2014/060641 filed on Apr. 11, 2014 and European Patent Application No. 13002337.7 filed on May 2, 2013.

FIELD OF THE INVENTION

The present invention relates to a method for controlling operation of a vapour compression system comprising two or more refrigeration entities, each refrigeration entity comprising a refrigerated volume. The vapour compression system being operated in accordance with the method of the invention is connected to a smart grid and is capable of responding to demands of the smart grid to increase or decrease power consumption.

BACKGROUND

It is sometimes desirable to control overall power consumption of energy consumers connected to a power grid in order to allow the power produced to the power grid to be utilised in an efficient manner, and in order to avoid adjusting the power production to short time scale variations in the power consumption. To this end so-called smart grids may request that the power consumers connected to the grid increase their power consumption when the total power production is higher than the total power consumption, e.g. during off-peak periods, and to decrease their power consumption when the total power production is lower than the total power consumption, e.g. during peak periods. Thereby the power consumption is adjusted to match the power production, rather than adjusting the power production to match the power consumption.

US 2010/0101254 A1 discloses a refrigerator comprising a fresh food compartment and a freezer compartment and one or more power consuming features or functions including a refrigeration system for cooling the fresh food compartment and the freezer compartment. A controller is configured to receive and process a signal indicative of current state of an associated energy supplying utility. The controller operates the refrigerator in one of a plurality of operating modes, including at least a normal operating mode and an energy savings mode, in response to the received signals. The controller may be configured to increase a setpoint temperature of the fresh food compartment and/or the freezer compartment in the energy savings mode.

SUMMARY

It is an object of embodiments of the invention to provide a method for controlling operation of a vapour compression system which allows the vapour compression system to interact with a smart grid without requiring additional hardware or components in the vapour compression system.

It is a further object of embodiments of the invention to provide a method for controlling operation of a vapour compression system in which interaction between the vapour compression system and a smart grid is established in an easy manner.

The invention provides a method for controlling operation of a vapour compression system, the vapour compression system comprising one or more compressors, a heat rejecting heat exchanger unit, a central controller, and two or more refrigeration entities, each refrigeration entity comprising an expansion device, an evaporator, a refrigerated volume arranged at the evaporator and a local controller arranged to control operation of the refrigeration entity in order to maintain a setpoint temperature in the refrigerated volume, each local controller being arranged to communicate with the central controller, the method comprising the steps of:

-   -   the central controller receiving a signal representing a         reference power consumption of the vapour compression system,     -   the central controller comparing the reference power consumption         to an actual power consumption of the vapour compression system,     -   the central controller communicating the result of said         comparison to each of the local controllers,     -   each local controller calculating a setpoint temperature for the         corresponding refrigerated volume, based on the result of the         comparison, and in order to obtain a power consumption of the         vapour compression system which is equal to the reference power         consumption, and     -   controlling operation of each refrigeration entity in accordance         with the calculated setpoint temperatures.

In the present context the term ‘vapour compression system’ should be interpreted to mean any system in which a flow of fluid medium, such as refrigerant, circulates and is alternatingly compressed and expanded, thereby providing either refrigeration or heating of a volume. Thus, the vapour compression system may be a refrigeration system, an air condition system, a heat pump, etc.

The vapour compression system being controlled by means of the method according to the present invention comprises one or more compressors, e.g. arranged in a compressor rack, a heat rejecting heat exchanger unit, a central controller, and two or more refrigeration entities. The heat rejecting heat exchanger unit may comprise one or more heat rejecting heat exchangers, e.g. in the form of one or more condensers or in the form of one or more gas coolers. In the heat rejecting heat exchanger unit heat is rejected from refrigerant flowing in a refrigerant path of the vapour compression system, e.g. to ambient air, or to another medium, such as brine, which could, e.g., be used in a chiller or a heat recovery system.

The central controller handles the overall control of the vapour compression system.

In the present context the term ‘refrigeration entity’ should be interpreted to mean an entity which provides cooling to a separate volume. The refrigeration entities may, e.g., be display cases in a supermarket, cool storage rooms, e.g. in a supermarket, a restaurant or a slaughterhouse, etc. The refrigeration entities may be adapted to cool goods to a chilled temperature, such as approximately 5° C., in which case the refrigeration entities operate as refrigerators. As an alternative, the refrigeration entities may be adapted to cool goods to a freezing temperature, such as approximately −18° C. or lower, in which case the refrigeration entities operate as freezers. As another alternative, some of the refrigeration entities may operate as refrigerators, while other refrigeration entities operate as freezers. This may, e.g., be relevant in supermarkets, where some kinds of goods need chilling while other kinds of goods need freezing.

Each refrigeration entity comprises an expansion device, e.g. in the form of an expansion valve, an evaporator and a refrigerated volume arranged at the evaporator. Refrigerant flowing in a refrigerant path is expanded by means of the expansion device, and the expanded refrigerant is supplied to the evaporator. In the evaporator, at least part of the refrigerant evaporates, thereby consuming energy and providing cooling for a volume arranged at the evaporator, i.e. the refrigerated volume.

Each refrigeration entity further comprises a local controller arranged to control operation of the refrigeration entity in order to maintain a setpoint temperature in the refrigerated volume. Accordingly, the temperature inside the refrigerated volume of each refrigeration entity is controlled by the local controller of the refrigeration entity in question, and the refrigeration entities are controlled independently of each other.

The local controllers are arranged to communicate with the central controller. Accordingly, each local controller may receive and/or supply signals from/to the central controller. This will be described further below.

Thus, the vapour compression system being controlled in accordance with the method of the invention may operate in the following manner. Refrigerant is compressed by means of the compressor(s), and the compressed refrigerant is supplied to the heat rejecting heat exchanger unit, where heat is rejected from the refrigerant. The refrigerant flow is then split in such a manner that parallel refrigerant flows are supplied to the refrigeration entities. In each refrigeration entity, refrigerant is expanded in the expansion device, and the expanded refrigerant is supplied to the evaporator, where the refrigerant is at least partly evaporated, thereby providing cooling to the refrigerated volume. Finally, the refrigerant leaving the evaporators of the refrigeration entities is supplied to the compressor(s).

According to the method of the present invention, the central controller initially receives a signal representing a reference power consumption of the vapour compression system. The reference power consumption may, e.g., represent a desirable power consumption of the vapour compression system. The reference power consumption signal may, e.g., be generated by an aggregator, e.g. a power grid supplying power to the vapour compression system or an entity representing the power grid. In this case the reference power consumption may represent a power consumption level which the power grid wishes the vapour compression system to meet. For instance, in the case that the power production of the power grid is higher than the total power consumption of power consuming entities connected to the power grid, the power grid may request some of the power consuming entities, such as the vapour compression system, to operate at a relatively high power consumption, in order to obtain that the total power consumption matches the power production. Similarly, in the case that the power production of the power grid is lower than the total power consumption of power consuming entities connected to the power grid, the power grid may request that some of the power consuming entities, such as the vapour compression system, to operate at a relatively low power consumption, in order to obtain that the total power consumption matches the power production.

As an alternative, the reference power consumption signal may be generated by the vapour compression system. In this case the reference power consumption signal may, e.g., be generated on the basis of information received from the power grid, e.g. requesting the vapour compression system to increase or decrease the power consumption, similar to the situation described above.

Next the central controller compares the reference power consumption to an actual power consumption of the vapour compression system. This comparison reveals whether or not the actual power consumption of the vapour compression matches the reference power consumption, or whether the actual power consumption of the vapour compression system is higher than or lower than the reference power consumption.

Next the central controller communicates the result of the comparison to each of the local controllers. Thus, the result of the comparison is made available to each of the local controllers, i.e. to the controllers which control operation of the individual refrigeration entities, including controlling the temperature inside the refrigerated volumes.

Upon receipt of this information, each local controller calculates a setpoint temperature for the corresponding refrigerated volume. The calculation of the setpoint temperature is based on the result of the comparison, and in order to obtain a power consumption of the vapour compression system which is equal to the reference power consumption. Accordingly, if the comparison reveals that the actual power consumption is lower than the reference power consumption, then the local controllers will calculate setpoint temperatures which increase the power consumption of the refrigeration entities. Similarly, if the comparison reveals that the actual power consumption is higher than the reference power consumption, then the local controllers will calculate setpoint temperatures which decrease the power consumption of the refrigeration entities.

Finally, the operation of each of the refrigeration entities is controlled in accordance with the calculated setpoint temperatures. Thereby the vapour compression system is operated in such a manner that the power consumption of the vapour compression system matches the reference power consumption.

It is an advantage that the setpoint temperatures are calculated by the local controllers, because each of the local controllers ‘knows’ the dynamics of the refrigeration entity which it controls. Thereby it is relatively easy for a local controller to calculate a setpoint temperature for the corresponding refrigeration entity. On the other hand, if the central controller should calculate setpoint temperatures for each of the refrigeration entities, the central controller would have to gain detailed knowledge of the dynamics of all the refrigeration entities, and the calculations would become relatively complex, and the result may be relatively inaccurate. Accordingly, the method of the present invention allows the vapour compression system to adjust its power consumption to match a reference power consumption in an easy manner, without requiring complex calculations, and without requiring additional hardware in the vapour compression system.

The step of comparing the reference power consumption to an actual power consumption of the vapour compression system may comprise obtaining an error signal, e, and the step of communicating the result of the comparison to each of the local controllers may comprise communicating the error signal, e, to each of the local controllers. The numerical value of the error signal, e, may advantageously represent how far the actual power consumption is from being equal to the reference power consumption, and the sign of the error signal, e, may advantageously indicate whether the actual power consumption is higher than or lower than the reference power consumption.

The step of calculating a setpoint temperature may comprise calculating a setpoint adjustment, ΔT, and adding said setpoint adjustment, ΔT, to a nominal setpoint temperature, T₀, thereby obtaining a calculated setpoint temperature, T_(ref)=T₀+ΔT. According to this embodiment, the calculated setpoint temperature is calculated with reference to a nominal setpoint temperature, which may represent a setpoint temperature which is desirable if no constraints were put on the power consumption of the vapour compression system. For instance, in the case that the refrigeration entity is a display case for chilling goods in a supermarket, the nominal setpoint temperature, T₀, may be 5° C. However, it may be acceptable to store the goods of the display case at 4° C. or 6° C., or even at 3° C. or 7° C. without risking damage to the goods. Accordingly, it may be acceptable to increase or decrease the setpoint temperature relative to the nominal setpoint temperature of this display case by 1° C. or 2° C., if the vapour compression system is required to decrease or increase its power consumption. In some systems the setpoint temperature may be allowed to vary within an even larger temperature interval. For instance, in case of freezers the temperature setpoint may be allowed to vary within the temperature interval from −18° C. to −25° C., or even to −30° C., if needed.

According to this embodiment constraints may be put on ΔT in order to ensure that the calculated setpoint temperature, T_(ref), does not deviate too much from the nominal setpoint temperature, T₀. Thereby it may be ensured that the temperature inside a given refrigerated volume is not allowed to exceed upper and lower limits which safeguard the quality and/or lifetime of the goods stored in the refrigerated volume.

The step of calculating a setpoint temperature may comprise decreasing a previous setpoint temperature in the case that the reference power consumption is higher than the actual power consumption of the vapour compression system, and increasing a previous setpoint temperature in the case that the reference power consumption is lower than the actual power consumption of the vapour compression system.

When cooling goods in a vapour compression system, the energy consumption is relatively high when the temperature of the goods is decreasing, and lower when the temperature of the goods is maintained at a substantially constant level, and even lower when the temperature of the goods is allowed to increase. In the case that the reference power consumption is higher than the actual power consumption, then the actual power consumption must be increased in order to match the actual power consumption to the reference power consumption. This may be obtained by decreasing a previous setpoint temperature. Thereby the temperature inside the refrigerated volume is decreased, thereby increasing the power consumption for a period of time until the temperature of the goods being stored in the refrigerated volume has reached the new setpoint temperature. If, at a later point in time, the reference power consumption becomes lower than the actual power consumption, and the actual power consumption therefore must be decreased, the setpoint temperature can once again be increased. Thereby the temperature inside the refrigerated volume is increased, thereby decreasing the power consumption. Thus, energy is ‘stored’ in the vapour compression system in the form of a reduced temperature of the stored goods, when an increased energy consumption is required, and the ‘stored’ energy is released again by allowing the temperature of the stored goods to increase, when a decreased energy consumption is required. Accordingly, the energy consumption of the vapour compression system can be shifted in time in order to meet demands of the power grid, while ensuring that the temperature of the stored goods is kept within specified temperature limits.

The method may further comprise the step of, for each refrigeration entity, defining an upper temperature limit, T_(max), and a lower temperature limit, T_(min), where T_(min)<T_(max), and wherein the step of calculating a setpoint temperature comprises limiting the calculated setpoint temperature to be within the interval T_(min) to T_(max). According to this embodiment, the setpoint temperature is not allowed to fall below T_(min) or to exceed T_(max), even if the calculated setpoint temperature falls outside these limits. If the power consumption of the vapour compression system is to be adjusted to match the reference power consumption, it may require that the setpoint temperature of one or more of the refrigeration entities is increased or decreased to a level which is unacceptable to the goods being stored. However, this must be avoided, and therefore the setpoint temperature is limited as described above, even if it has the consequence that the power consumption of the vapour compression system is not matched to the reference power consumption, or if it has the consequence that a longer time is allowed to lapse before the power consumption of the vapour compression system is matched to the reference power consumption.

The step of limiting the calculated setpoint temperature may be performed using a saturation filter.

The step of defining an upper temperature limit, T_(max), and a lower temperature limit, T_(min), may comprise the steps of:

-   -   obtaining a measure for a temperature of goods being stored in         the refrigerated volume, and     -   calculating the upper temperature limit, T_(max), and the lower         temperature limit, T_(min), on the basis of the obtained measure         for a temperature of goods being stored in the refrigerated         volume.

According to this embodiment, the temperature limits are calculated on the basis of the temperature of the goods being stored, rather than on the basis of the air temperature inside the refrigerated volume. For instance, if the temperature of the stored goods is relatively low, the upper temperature limit may, temporarily, be at a relatively high level without risking damage to the stored goods.

The measure for the temperature of the stored goods may be obtained in any suitable manner, such as by measuring the temperature or by estimating the temperature.

The method may further comprise the steps of, for each refrigeration entity:

-   -   calculating an error signal, e_(s), reflecting a difference         between a calculated setpoint temperature and a limited setpoint         temperature,     -   supplying the calculated error signal, e_(s), to the local         controller, and     -   the local controller modifying the calculated setpoint         temperature based on the calculated error signal, e_(s).

According to this embodiment, the calculated error signal, e_(s), reflects the difference between the calculated setpoint temperature, i.e. the setpoint temperature which is desirable in order to obtain an actual power consumption which is equal to the reference power consumption, and the limited setpoint temperature, i.e. the setpoint temperature which must be selected in order to ensure that the temperature inside the refrigerated volume remains within an acceptable temperature range. Accordingly, the calculated error signal, e_(s), reflects how much the desired setpoint temperature must be adjusted in order to ensure that the temperature inside the refrigerated volume remains within an acceptable temperature range. Taking this into account when the setpoint temperature is selected furthermore ensures that the setpoint temperature is not allowed to differ too much from a nominal setpoint temperature, i.e. a setpoint temperature which would be applied if no constraints with regard to power consumption were applied. Thereby it is ensured that the operation of the refrigeration entity can quickly be changed to an operation in accordance with the nominal setpoint temperature in the case that operating conditions change. Therefore this embodiment may be referred to as including an ‘anti-wind up feature’.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawings in which

FIG. 1 is a diagrammatic view of a vapour compression system which is adapted to be controlled using a method according to a first embodiment of the invention,

FIG. 2 is a diagrammatic view of a local setpoint controller for a vapour compression system which is adapted to be controlled using a method according to a second embodiment of the invention, and

FIG. 3 is a diagrammatic view of a local setpoint controller for a vapour compression system which is adapted to be controlled using a method according to a third embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 is a diagrammatic view of a vapour compression system 1 which is adapted to be controlled using a method according to a first embodiment of the invention. The vapour compression system 1 comprises one or more compressors, a heat rejecting heat exchanger unit and a central controller, all of which are contained in the box 2 denoted ‘refrigeration system’, but not shown in detail in FIG. 1.

The vapour compression system 1 further comprises a number of refrigeration entities, each comprising an expansion device, an evaporator and a refrigerated volume, which are not illustrated in FIG. 1. Each refrigeration entity further comprises a local setpoint controller 3, three of which are illustrated in FIG. 1.

The vapour compression system 1 of FIG. 1 may, e.g., be operated in the following manner. A signal representing a reference power consumption, {dot over (W)}_(ref), is supplied to the central controller, and received at comparing unit 4. The reference power consumption, {dot over (W)}_(ref), represents a power consumption which it is desirable that the vapour compression system 1 operates at. As described above, the reference power consumption signal may be generated by a power grid which supplies power to the vapour compression system 1, and it may reflect a power consumption level which allows the power grid to control the power consumption of power consumers connected to the power grid to match the power production of the power grid.

The central controller further supplies a signal representing the actual power consumption, {dot over (W)}_(comp), of the vapour compression system 1 to the comparing unit 4. Thus, in the comparing unit 4 the actual power consumption, {dot over (W)}_(comp), of the vapour compression system 1 is compared to the desired power consumption level, i.e. to the reference power consumption, {dot over (W)}_(ref). Based on this, an error signal is generated and supplied to each of the local setpoint controllers 3.

Each of the local setpoint controllers 3 comprises a proportional integral (PI) controller 5 and a saturation filter 6. The error signal generated at the comparing unit 4 is supplied to each of the PI controllers 5, and based on the error signal, each PI controller 5 calculates a setpoint adjustment, ΔT_(calc), in such a manner that an adjustment of the setpoint temperature of the corresponding refrigerated volume is expected to cause the actual power consumption of the vapour compression system 1 to become equal to the reference power consumption, {dot over (W)}_(ref).

The calculated setpoint adjustment, ΔT_(calc), is then supplied to the saturation filter 6. The output of the saturation filter 6, ΔT_(i), is equal to ΔT_(calc) if T_(0,i)+ΔT_(calc) is within a predefined range defined by a lower temperature limit, T_(min), and an upper temperature limit, T_(max), where T_(0,i) is a nominal setpoint temperature for the i'th refrigeration entity. If T_(0,i)+ΔT_(calc)<T_(min), then ΔT_(i)=T_(min)−T_(0,i), and if T_(0,i)+ΔT_(calc)>T_(max), then ΔT_(i)=T_(max)−T_(0,i). The nominal temperature, T_(0,i), the lower temperature limit, T_(min), and/or the upper temperature limit, T_(max), may vary from one refrigeration entity to the other.

The setpoint adjustment, ΔT_(i), obtained for each refrigeration entity in the manner described above is then supplied to an addition unit 7, where the setpoint adjustment, ΔT_(i), is added to the nominal setpoint temperature, T_(0,i), for the refrigeration entity to obtain a calculated setpoint temperature, T_(ref,i)=T_(0,i)+ΔT_(i). The calculated setpoint temperature, T_(ref,i), is then supplied to the refrigeration system 2, and each refrigeration entity is controlled in order to obtain a temperature inside the corresponding refrigerated volume, which is equal to the calculated setpoint temperature, T_(ref,i).

Furthermore, each local setpoint controller 3 comprises a comparing unit 8, which receives a signal representing ΔT_(calc) and a signal representing ΔT_(i). The comparing unit 8 generates an error signal, e_(s,i), representing the difference between ΔT_(calc) and ΔT_(i). Thus, the error signal, e_(s,i), reflects how much it is necessary to adjust the calculated setpoint adjustment, ΔT_(calc) in order to ensure that the temperature inside the refrigerated volume remains within an acceptable temperature range.

The error signal, e_(s,i), is fed back to the PI controller 5. Thereby the error signal, e_(s,i), is taken into account in subsequent calculations of ΔT_(calc).

FIG. 2 is a diagrammatic view of a local setpoint controller 3 for a vapour compression system which is adapted to be controlled using a method according to a second embodiment of the invention. The vapour compression system may, e.g., be the vapour compression system illustrated in FIG. 1.

The local setpoint controller 3 comprises a saturation filter 6 and a comparing unit 8 as described above. The local setpoint controller 3 further comprises a first gain unit 9 and a second gain unit 10. An error signal, {dot over (W)}_(ref)-{dot over (W)}_(comp), where {dot over (W)}_(ref) is a reference power consumption and {dot over (W)}_(comp) is an actual power consumption of the vapour compression system, as described above, is supplied to the first gain unit 9 and to the second gain unit 10. In the first gain unit 9, the error signal, e, is multiplied by a gain factor, K. The resulting signal represents a setpoint adjustment to be applied to the setpoint temperature of the corresponding refrigeration entity, and is supplied to a first adding unit 11.

In the second gain unit 10, the error signal, e, is multiplied by a gain factor,

$\frac{K}{T_{i}},$

where T_(i) is an integrator time constant. The output of the second gain unit 10 is supplied to a second adding unit 12.

An error signal, e_(s), is calculated in the manner described above, based on the input to and the output from the saturation filter 6. The error signal, e_(s), is supplied to a third gain unit 13, and the output of the third gain unit 13 is supplied to the second adding unit 12. Thus, in the second adding unit 12 the output of the second gain unit 10 and the output of the third gain unit 13 are added. It should be noted that as long as the calculated setpoint adjustment is within the specified range, i.e. as long as the setpoint adjustment is not limited by the saturation filter, the error signal, e_(s), is zero, and thereby the contribution from the third gain unit 13 is also zero.

The output of the second adding unit 12 is supplied to an integrator unit 14, and the output of the integrator unit 14 is supplied to the first adding unit 11. In the integrator unit 14 the error signal, e_(s), over time is essentially integrated, and the output of the integrator unit 14 thereby represents the combined adjustment, by the saturation filter 6, of the calculated setpoint adjustment over time. Since the output of the integrator unit is supplied to the first adding unit 11, this is taken into account when the calculated setpoint adjustment, ΔT_(calc), is calculated and supplied to the saturation filter 6.

In the embodiment illustrated in FIG. 2 the setpoint temperature is prevented from deviating too much from the nominal setpoint temperature. Thereby it is ensured that the operation of the refrigeration entity can quickly be returned to being operated in accordance with the nominal setpoint temperature in the case that operating conditions change. This may be referred to as an ‘anti-wind up’ feature.

FIG. 3 is a diagrammatic view of a local setpoint controller 3 for a vapour compression system being adapted to be controlled using a method according to a third embodiment of the invention. The vapour compression system may, e.g., be the vapour compression system illustrated in FIG. 1. The local setpoint controller 3 comprises a PI controller 5, a saturation filter 6 and a comparing unit 8 as described above with reference to FIG. 1. The local setpoint controller 3 operates essentially as described above with reference to FIGS. 1 and 2, and it will therefore not be described in further detail here.

The local setpoint controller 3 of FIG. 3 further receives a temperature signal, T_(food), reflecting the temperature of goods being stored in the refrigerated volume of the refrigeration entity being controlled by the local setpoint controller 3. The temperature, T_(food), may, e.g., be obtained by a direct measurement of the temperature, or it may be estimated.

The temperature, T_(food), is processed by means of an adaptive algorithm, and the result is supplied to the saturation filter 6, where it is used for calculating the upper and lower limits being applied by the saturation filter 6. The upper limit may, e.g., be calculated in the following manner.

u _(max)(t)=U _(max) +K _(u)(T _(max) −T _(food)(t)),

where u_(max)(t) is the upper limit as a function of time, K_(u) is a proportional gain, T_(max) is the upper limit of the temperature of the goods being stored in the refrigerated volume, T_(food)(t) is the measured or estimated temperature of the stored goods as a function of time, and U_(max)=(T_(max)−T₀), where T₀ is a nominal temperature setpoint.

Similarly, the lower limit may, e.g., be calculated in the following manner.

u _(min)(t)=U _(min) +K _(l)(T _(min) −T _(food)(t)),

where u_(min)(t) is the lower limit as a function of time, K_(l) is a proportional gain, T_(min) is the lower limit of the temperature of the goods being stored in the refrigerated volume, T_(food)(t) is the measured or estimated temperature of the stored goods as a function of time, and U_(min)=(T_(min)−T₀), where T₀ is a nominal temperature setpoint.

Thus, according to this embodiment, the upper and lower limits applied by the saturation filter 6 change as a function of time, depending on the temperature of the stored goods. This improves the performance of the control strategy.

While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure. 

What is claimed is: 1-8. (canceled)
 9. A method for controlling operation of a vapour compression system, the vapour compression system comprising one or more compressors, a heat rejecting heat exchanger unit, a central controller, and two or more refrigeration entities, each refrigeration entity comprising an expansion device, an evaporator, a refrigerated volume arranged at the evaporator and a local controller arranged to control operation of the refrigeration entity in order to maintain a setpoint temperature in the refrigerated volume, each local controller being arranged to communicate with the central controller, the method comprising the steps of: for each refrigeration entity, defining an upper temperature limit, T_(max), and a lower temperature limit, T_(min), where T_(min)<T_(max), the central controller receiving a signal representing a reference power consumption of the vapour compression system, the central controller comparing the reference power consumption to an actual power consumption of the vapour compression system, the central controller communicating the result of said comparison to each of the local controllers, each local controller calculating a setpoint temperature for the corresponding refrigerated volume, based on the result of the comparison, and in order to obtain a power consumption of the vapour compression system which is equal to the reference power consumption, the calculated setpoint temperature being limited to be within the interval T_(min) to T_(max), and controlling operation of each refrigeration entity in accordance with the calculated setpoint temperatures.
 10. The method according to claim 9, wherein the step of comparing the reference power consumption to an actual power consumption of the vapour compression system comprises obtaining an error signal, e, and wherein the step of communicating the result of the comparison to each of the local controllers comprises communicating the error signal, e, to each of the local controllers.
 11. The method according to claim 9, wherein the step of calculating a setpoint temperature comprises calculating a setpoint adjustment, ΔT, and adding said setpoint adjustment, ΔT, to a nominal setpoint temperature, T₀, thereby obtaining a calculated setpoint temperature, T_(ref)=T₀+ΔT.
 12. The method according to claim 9, wherein the step of calculating a setpoint temperature comprises decreasing a previous setpoint temperature in the case that the reference power consumption is higher than the actual power consumption of the vapour compression system, and increasing a previous setpoint temperature in the case that the reference power consumption is lower than the actual power consumption of the vapour compression system.
 13. The method according to claim 9, wherein the step of limiting the calculated setpoint temperature is performed using a saturation filter.
 14. The method according to claim 9, wherein the step of defining an upper temperature limit, T_(max), and a lower temperature limit, T_(min), comprises the steps of: obtaining a measure for a temperature of goods being stored in the refrigerated volume, and calculating the upper temperature limit, T_(max), and the lower temperature limit, T_(min), on the basis of the obtained measure for a temperature of goods being stored in the refrigerated volume.
 15. The method according to claim 9, further comprising the steps of, for each refrigeration entity: calculating an error signal, e_(s), reflecting a difference between a calculated setpoint temperature and a limited setpoint temperature, supplying the calculated error signal, e_(s), to the local controller, and the local controller modifying the calculated setpoint temperature based on the calculated error signal, e_(s).
 16. The method according to claim 10, wherein the step of calculating a setpoint temperature comprises calculating a setpoint adjustment, ΔT, and adding said setpoint adjustment, ΔT, to a nominal setpoint temperature, T₀, thereby obtaining a calculated setpoint temperature, T_(ref)=T₀+ΔT.
 17. The method according to claim 10, wherein the step of calculating a setpoint temperature comprises decreasing a previous setpoint temperature in the case that the reference power consumption is higher than the actual power consumption of the vapour compression system, and increasing a previous setpoint temperature in the case that the reference power consumption is lower than the actual power consumption of the vapour compression system.
 18. The method according to claim 11, wherein the step of calculating a setpoint temperature comprises decreasing a previous setpoint temperature in the case that the reference power consumption is higher than the actual power consumption of the vapour compression system, and increasing a previous setpoint temperature in the case that the reference power consumption is lower than the actual power consumption of the vapour compression system.
 19. The method according to claim 10, wherein the step of limiting the calculated setpoint temperature is performed using a saturation filter.
 20. The method according to claim 11, wherein the step of limiting the calculated setpoint temperature is performed using a saturation filter.
 21. The method according to claim 12, wherein the step of limiting the calculated setpoint temperature is performed using a saturation filter.
 22. The method according to claim 10, wherein the step of defining an upper temperature limit, T_(max), and a lower temperature limit, T_(min), comprises the steps of: obtaining a measure for a temperature of goods being stored in the refrigerated volume, and calculating the upper temperature limit, T_(max), and the lower temperature limit, T_(min), on the basis of the obtained measure for a temperature of goods being stored in the refrigerated volume.
 23. The method according to claim 11, wherein the step of defining an upper temperature limit, T_(max), and a lower temperature limit, T_(min), comprises the steps of: obtaining a measure for a temperature of goods being stored in the refrigerated volume, and calculating the upper temperature limit, T_(max), and the lower temperature limit, T_(min), on the basis of the obtained measure for a temperature of goods being stored in the refrigerated volume.
 24. The method according to claim 12, wherein the step of defining an upper temperature limit, T_(max), and a lower temperature limit, T_(min), comprises the steps of: obtaining a measure for a temperature of goods being stored in the refrigerated volume, and calculating the upper temperature limit, T_(max), and the lower temperature limit, T_(min), on the basis of the obtained measure for a temperature of goods being stored in the refrigerated volume.
 25. The method according to claim 13, wherein the step of defining an upper temperature limit, T_(max), and a lower temperature limit, T_(min), comprises the steps of: obtaining a measure for a temperature of goods being stored in the refrigerated volume, and calculating the upper temperature limit, T_(max), and the lower temperature limit, T_(min), on the basis of the obtained measure for a temperature of goods being stored in the refrigerated volume.
 26. The method according to claim 10, further comprising the steps of, for each refrigeration entity: calculating an error signal, e_(s), reflecting a difference between a calculated setpoint temperature and a limited setpoint temperature, supplying the calculated error signal, e_(s), to the local controller, and the local controller modifying the calculated setpoint temperature based on the calculated error signal, e_(s).
 27. The method according to claim 11, further comprising the steps of, for each refrigeration entity: calculating an error signal, e_(s), reflecting a difference between a calculated setpoint temperature and a limited setpoint temperature, supplying the calculated error signal, e_(s), to the local controller, and the local controller modifying the calculated setpoint temperature based on the calculated error signal, e_(s).
 28. The method according to claim 12, further comprising the steps of, for each refrigeration entity: calculating an error signal, e_(s), reflecting a difference between a calculated setpoint temperature and a limited setpoint temperature, supplying the calculated error signal, e_(s), to the local controller, and the local controller modifying the calculated setpoint temperature based on the calculated error signal, e_(s). 